1. Field of the Invention
The present invention generally relates to mechanisms for equalizing the movement of opposing members toward a workpiece and the pressure applied by the members to opposite sides of the workpiece. More specifically, this invention relates to an equalizing mechanism for a welding gun suitable for use on automated assembly lines, wherein the mechanism equalizes the movement of a pair of electrode tips toward a workpiece and thereafter the pressure applied by the electrode tips to opposite sides of the workpiece while the workpiece is being welded.
2. Description of the Prior Art
Electric welding guns, and particularly resistance welding guns, are widely used in mass production to join workpieces to form permanent assemblies. Resistance welding guns are especially suited for use on automated automobile assembly lines where body panels, such as doors and hoods, and their support structures are welded together to form subassemblies, which are then welded to the frame or unit construction of an automobile. Generally, resistance welding is a process that, through the application of heat and pressure, coalesces two or more workpieces, such as metal sheets or panels, without the use of fluxes or filler metals to form a permanent joint. The necessary heat is generated by the resistance of the workpieces to the flow of an electric current between two electrodes that serve as terminals for an electrical circuit. During the welding process, the electrodes contact opposite sides of workpieces to be welded so as to firmly clamp the workpieces together.
The application of force through the electrodes is beneficial in that it assures a firm contact between the workpieces being welding, and also assures that a sufficient electrical contact between the workpieces and the electrodes is made. However, excessive force must be avoided where the workpieces being welded are thin and, therefore, susceptible to deformation from excessive impact loads. This is particularly true in the automotive industry where the demand for lighter-weight vehicles has resulted in the use of thinner sheet metal. In view of the high forces that must typically be applied to ensure sufficient contact between the electrodes and the sheet metal, deformation often becomes a recurring problem on automobile assembly lines.
Welding guns typically used on automated assembly lines include a pair of arms, at least one of which is pivotably mounted to a base, with each arm being fitted with an electrode. Where each arm is pivotably mounted, a cylinder is often attached to the opposite end of each arm to rotate the arms relative to each other at a predetermined rate. If only one arm is pivotably mounted, as disclosed in U.S. Pat. No. 3,299,247 to Waltonen, a camming device may be employed to control the orientation of the moving electrode so that the final motion of the moving electrode is roughly linear toward the stationary electrode to accommodate electrode deterioration and variances in workpiece thickness. In each case, one of the electrodes will typically come in contact with the workpiece prior to the second electrode, such that the workpiece is at least initially subject to unequal loading by the electrodes. While low asymmetric loading does not typically cause excessive deformation during assembly operations with thick workpieces, moderate asymmetric loads can cause unacceptable deformation of relatively thin workpieces, such as automotive body panels.
Several approaches for minimizing asymmetric clamping forces applied by welding gun electrodes have been suggested in the prior art. An early approach taught by U.S. Pat. No. 1,980,228 to Rogers was to provide adjustment to the force applied by a pair of compression springs. Rogers disclosed an eccentric pin which, when rotated, adjusted the gap between a pair of electrodes when the electrodes were subject to the force of the springs urging the electrodes together. However, the welding gun taught by Rogers is not well suitable for use on an automated assembly line in that the actual force imposed by the electrodes on the workpiece depends upon possible variations in thicknesses of the workpiece. Reliance on springs for force equalization on a workpiece is also taught in U.S. Pat. Nos. 3,008,032 and 3,008,034 to Wolfbauer, Jr., and U.S. Pat. No. 4,549,457 to Bloch, each of which further incorporates a camming device to rotate a tool and anvil into position on opposite sides of a workpiece. Notably, considerable force is applied to the cam rollers and tracks of U.S. Pat. No. 3,008,034 during welding when the workpiece is clamped between the electrodes, with the potential for significantly shortening the service lives of the rollers and tracks.
An approach taught by U.S. Pat. No. 5,036,175 to Umeda relies on a spring and cylinder combination to equalize the force imposed by two welding electrodes, one of which is mounted on a rotating arm while the other is mounted on a linearly moving arm. Umeda employs an intermediate linkage between the arms to urge the linearly moving arm away from the workpiece under the opposing force of the spring operating on a sliding portion of the arm. As the cylinder begins to move the rotating arm into position for welding, the linkage gradually releases the linearly moving arm, allowing it to come into contact with the workpiece under the force imposed by the spring. Without a camming device, the path of the rotating arm does not include a linear portion that accommodates electrode deterioration and variances in workpiece thickness.
The durability of a welding gun and other clamping-type devices that employ similarly moving arms depends in part on the wear of the components, and particularly the pivots, sliders and rollers that maintain the proper orientation of the moving components throughout the operating cycle of the device. In particular, side loads on pivot pins and cam rollers and nonaxial rotational loads on sliding members lead to premature failure of these components. It can be appreciated that each of the prior art resistance welding guns discussed above achieves certain operational advantages. However, the components that provide such advantages can negatively affect the durability of the welding gun. For example, Umeda provides for equalization of the electrode clamping load with a spring-loaded sliding member that is subject to high rotational (nonaxial) loading. The durability of Umeda's welding gun is dependent in part on the ability of the device to repeatedly withstand rotational loads imposed on the device each time at least one rotating electrode is clamped onto workpieces, during which the load applied by the rotating electrode must be prevented from overloading the components that provide for force and movement equalization. As another example, Waltonen provides the advantage of preventing high loads from being applied to the camming device that controls the movement and orientation of a moving electrode. However, the construction of the disclosed welding gun is prone to mispositioning of the electrodes relative to each other and the workpiece due to twisting of the structural members under high application loads, which is detrimental to the pins and bearings that hold the assembly together.
Accordingly, what is needed is a durable apparatus that provides equalized movement and loading of a pair of contacts, such as resistance welding electrodes, brought into contact with a workpiece, such that the loads imposed by the electrodes on the workpiece will be substantially equal and will not overload the mechanism providing movement and force equalization.